What Are Carrier Selective Contacts?

Increasing the efficiency in solar cells

In solar cells, carrier selective contacts (CSCs) are used to reduce contact recombination and thus reduce current losses. While recombination losses occur at the front contact, they are much more severe at the rear contact. This is because the rear surface is completely metallized while the front surface is only partly metallized. Of course, this is because there would be no power generation if the entire solar cell surface were covered.

Recombination occurs when electrons and holes essentially cancel each other out. If carriers cancel each other out, there are none to move around a circuit and generate a current. CSCs prevent this by blocking one carrier and accepting another. Hole selective contacts have very high work functions and are attached to the p or p+ side of the solar cell. Electron selective contacts have very low work functions and are attached to the n or n+ side of the solar cell. A high work function translates to a lower Fermi level, while a low work function translates to a higher Fermi level.

Some notable CSCs are TiO2 (electron), SiO2 (electron), and MoOx (hole). WOx (hole), and PEDOT (hole).

Case study: SiO2 as an electron selective contact

Here, SiO2 has a lower work function than the n-Si base and thus bends the conduction energy band edge of the n-Si down. The valence band edge of the SiO2 is so much lower than that of the n-Si that an insurmountable barrier is generated. This barrier is too high for holes to overcome and they bounce off of it.

The same thing happens with hole selective contacts except that they have high work functions.

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